Reverse osmosis and nanofiltration are one of the technologies that have recently gained attention in the water treatment field. Especially, processes using reverse osmosis are now coming into widespread use in the field of seawater desalination or sewage recycling.
According to a Global Water Intelligence special report entitled “Water Reuse Markets 2005-2015: A Global Assessment & Forecast”, global water reuse capacity is expected to grow from 2 million tons a day today to 5.4 million tons per day in 2015. Over the same period, global desalination capacity is expected to grow from 3 million tons per day to 6.2 million tons per day. Besides, reverse osmosis and nanofiltration are advanced water treatment technologies which are getting attention in the field of surface water and ground water treatment, industrial wastewater treatment, and zero-discharge recycling.
However, membrane fouling, i.e., filtration membrane fouling, has an obstacles to the commercialization or the on-site installation and operation of these membranes of reverse osmosis and nanofiltration. Membrane fouling refers to a situation in which foreign substances present in water flowing into a filtration membrane are deposited or adsorbed on the surface of the filtration membrane in a way that degrades the water permeability of the filtration membrane.
There are various types of foreign substances causing membrane fouling, including suspended particles, colloids, organic matters, microbes, and mineral salts such as calcium salts. It is considerably difficult to predict membrane fouling because of these various foreign substances causing membrane fouling.
In general, the SDI (Silt Density Index) is used to predict membrane fouling occurring in reverse osmosis or nanofiltration. The SDI is a measure of the rate at which membrane fouling will occur. The SDI is a method of measurement where influent water is passed through a 0.45 μm pore size filter with 47 mm diameter at a pressure of 30 psi to measure the degree of fouling of membranes by suspended solids (SS).
The initial time T0 to filter 500 ml of water is measured, and then the time T1 to filter 500 ml of water is measured again after 15 minutes. The ratio of T0 to T1 is used as a measure of membrane fouling.
The SDI measuring method is currently the most widely used method to predict the propensity for membrane fouling by influent water in reverse osmosis or nanofiltration. Typically, an SDI value less than 3 is considered to be acceptable, and an SDI value greater than 5 indicates that severe fouling will occur.
However, the above-described SDI has its limits in that it does not use the same situation occurring in a reverse osmosis membrane (RO membrane). That is, the SDI is an indirect method of evaluating the fouling potential of suspended particles larger than 0.45 μm on membranes. Therefore, the SDI cannot evaluate the effects of fine materials such as colloids or organic matters smaller than 0.45 μm.
Moreover, reverse osmosis and nanofiltration systems are operated in a cross-flow mode in which influent water flows in a direction orthogonal to the permeation direction of a filtration membrane, thus making it impossible to measure the surface characteristics of foulants, the main cause of membrane fouling, by the SDI. Accordingly, much research revealed that SDI measurements are different from actual results of operations.
Although MFI (Modified Fouling Index) can be used to compensate for these problems of the SDI, a limited range of membrane foulants are available for this method because the MFI and the SDI basically use the same filtration membrane.
To overcome this, methods such as MFI-UF (Modified fouling index—Ultrafilter) or MFI-NF (Modified fouling index—Nanofilter) were proposed. However, these methods cannot predict the propensity for various types of membrane fouling that may occur in a filtration membrane because they also use a single type of membrane.
In view of this, the present inventor suggested earlier-filed Korean Patent Registration No. 106901 titled ‘Device for measuring pollution index of membrane’. The device for measuring the pollution index of a membrane disclosed in the-above Korean Patent Registration is a device for measuring the pollution index of a membrane that predicts membrane fouling, in advance, caused by hydrophilic and hydrophobic particulate materials, colloids, organic matters, etc in a water treatment process using a reverse osmosis membrane or nanofiltration membrane. This device is characterized in that different types of filtration membranes such as hydrophilic microfiltration membranes, hydrophobic microfiltration membranes, and ultrafiltration membranes are combined together to quantify the intensity of membrane fouling by various types of membrane foulants present in influent water to be treated by reverse osmosis or nanofiltration.
The above-mentioned Korean Patent Registration is advantageous in that the measurement can be done simply due to parallel connection and the evaluation differs depending on the properties of membrane foulants, but disadvantageous in that it is difficult to separate different types of membrane fouling according to membrane foulants source and evaluate them.
In addition, the present inventor proposed earlier-filed Korean Unexamined Patent Application No. 2011-0089719 titled ‘Device for predicting pollution index of membrane’. In this Korean Unexamined Patent Application, different types of membranes foulants can be separated and evaluated by connecting a plurality of filtration membranes with different filtration characteristics in series. However, this patent application is disadvantageous in that three filtration membranes, three pumps, and three tanks are required and a relatively long period of time is needed for the measurement because of the serial structure.